Referring to the current flip-chip technique, electrode pads are located on a surface of a semiconductor integrated circuit (IC) chip and a corresponding conductive pad is provided on a circuit board. Solder bumps or other conductively adhesive materials are mounted on an active surface of the chip, which is provided on the top of the circuit board, such that the solder bump or the conductively adhesive material can be used as input/output connections for electrically and mechanically connecting the chip to the circuit board.
Referring to FIG. 1 which demonstrates a flip chip device known in the prior-art, a plurality of metal bumps 11 is formed on an electrode pad 12 of a chip 13, and a plurality of solder bumps 14 made of solder materials is formed on a conductive pad 15 of a circuit board 16. Under a reflow-soldering temperature condition in which the solder bump 14 can be melted, the solder bump 14 is subject to a reflow-soldering process to make contact with a corresponding metal bump 11, so as to form a solder joint 17. Referring to a solder bump joint, a bottom cement material 18 such as underfill can be further filled in a gap between the chip and the circuit board, such that a thermal expansion difference existed between the chip 13 and the circuit board 16 can be prevented and a stress of the solder joint 17 can be reduced. Subsequently, solder balls 19 are mounted on the other surface of the circuit board 16, such that the circuit board 16 integrated with the chip 13 can be electrically connected to an external electronic device.
Recently, the solder material is deposited on the conductive pad located on an upper surface of the circuit board to form the solder bump and the solder ball is formed on the conductive pad located on a lower surface of the circuit board by stencil printing technology. Referring to FIG. 2, the currently used stencil printing technology mainly provides a circuit board 20 formed with a solder mask layer 21 such as green paint on a surface thereof, and a plurality of conductive pads 22 for forming locations of solder materials (not shown) such as solder pastes. Firstly, a stencil 23 having a plurality of grids 23a is provide on the circuit board 20. After coating the stencil 23 with the solder material, a roller 24 is repeatedly rolled on the stencil 23 or a spraying process is performed, such that the solder material is able to form a solder (not shown) on the conductive pad using the gird 23a of the stencil 23 after the removal of the stencil 23. Subsequently, under a reflow-soldering temperature condition in which the solder can be melted, the solder is subject to a reflow-soldering process to form a pre-soldering bump on the conductive pad of the circuit board. Furthermore, the stencil of the foregoing stencil printing technology is preferably a steel plate.
During practical operation, along with the blooming development of various portable products in the fields of communication, networking and computing, packages such as ball grid array (BGA), flip chip, chip size package (CSP) and multi chip module (MCM) which are characterized with a miniaturized integrated circuit (IC) area, a high density and multiple leads have become the mainstream of the packaging market. Highly effective chips such as a microprocessor, a chip set and a drawing chip are usually combined with the foregoing package to achieve an operating function of a higher speed. However, circuits on a circuit board and a dimension of a conductive pad must be miniaturized for such structures to be applied. When the dimension and pitches of the conductive pad are miniaturized, dimensions of openings formed penetrating through the stencil also need to be reduced. Thus, a cost of stencil fabrication cannot be reduced due to difficulties in performing mold opening of the stencil. Also, it is difficult to allow the solder material to pass through the opening of the stencil due to smallness of the opening.
Furthermore, referring to criteria on the production accuracy of the solder material, frequencies and cleanness of stencil printing have to be concerned in addition to a precise dimension of the stencil while performing the stencil printing technology. As the number of printings is increased, the solder material remained in a wall of the opening of the stencil is as well accumulated. Since the solder material is of a certain viscosity, the amount and shape of the solder material for performing next printing will be different from the specification if the solder material is previously accumulated. Therefore, the stencil has to be wiped out after performing a certain number of printings to ensure the cleanness of the stencil during practical operation, otherwise the shape and dimension of the solder material might be altered to cause an inconvenience as well as a decreased reliability in fabrication.